Studies on oxychlorination of MoS 2 in a fluid bed reactor
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I.
INTRODUCTION
II.
T H E O R E T I C A L CONSIDERATION
THE principal sources of molybdenum can be broadly di-
A. Thermodynamics
vided in two: (i) primary molybdenite and (ii) base metal sulfides containing minor quantities of molybdenum known as secondary molybdenum resources. Since the secondary molybdenum resources are very lean in nature, a series of ore dressing operations is to be incorporated in the process flow sheet for concentrating the ore to technical grade MoS2. In India, secondary, low-grade molybdenite concentrate is obtained as a byproduct of the Ore Dressing Plant of Uranium Corporation of India (UCIL), Jaduguda (Bihar). This ore, apart from U3Os, contains minor quantities of molybdenum, nickel, and copper sulfides and these sulfides are collected from the bulk flotation cells of the plant. The mixed sulfide concentrates so obtained are subjected to a differential flotation to recover MoSs concentrate assaying about 30 pct Mo content. Further concentration to obtain technical grade ( ~ 96 pct) MoS2 by repeated flotation is not economical because in this process the overall efficiency goes down as low as 50 pct. Hence, it is necessary to stop at some stage in the middle and adopt an economically viable route to process this low grade M o S 2. Chlorination at low temperature using the technique of fluidization is one of the methods m developed in this laboratory. During the initial stages of investigations, difficulties such as heat dissipation, lump formation, slow reaction rate, materials of construction, etc., were encountered. The feasibility of any such process is a function of thermodynamics, kinetics, and material parameters. Hence, it was thought that some progress could be possible in the development of the chlorination technique, if additional information on the behavior of the various parameters of the system are made available. Accordingly, a program was initiated to study the kinetics of chlorination of pure molysulfide to determine the rate controlling step of the overall reaction. This paper reports the results of these investigations carried out in a fluidized bed reactor.
The reaction between MoSs and C12 in the presence as well as absence of O5 has been found to be thermodynamically feasible, t21 The two types of reactions are AG~298)= - 197.74ld/moleMoCI5
2MOS2 + 7C12 AHO(298)=_529.0kl/moleMoCl 5 > 2MOC15 + 2S2C12
[1] AG1298)= -732.5 kJ/moleMOO2C12> M o S 2 + C12 + 3 0 2 AH~(298)=_723.83kl/moleMoO2C12 MOO2C12 + 2SO 2
[2] The physico-chemical aspects of these above reactions may further be seen in the literature.t3-6] It is seen from the above reactions that the chlorination in the presence of 02 (Eq. [2]) is strongly favored. Moreover, the reaction product MoOzC12 can be reduced directly by H2 to micron size Mo powder or can be dissolved in water and subsequently processed to obtain commercially important intermediates such as MoO3 or calcium molybdate (CaMoO4).
B. Kinetics The rate of oxychlorination of MoS2 at any instant may be considered to be a function of the avail
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